1. Field of the Invention
The present invention relates to a dither pattern which can be used for image processing for forming an image on a print medium by using a plurality of printing heads for ejecting ink and a method for forming the dither pattern. Further, the present invention relates to an image printing method and an image printing apparatus for printing an image on a print medium by performing the image processing.
2. Description of the Related Art
In a serial type printing apparatus for printing an image by printing dots on a print medium by using a plurality of printing elements, multi-pass printing is often adopted to reduce density unevenness and stripes in a printed image. In multi-pass printing, an image which can be printed by one printing scan of a printing head is printed in a stepwise manner by a plurality of printing scans of the printing head. In this multi-pass printing, a print medium is conveyed between scans a distance which is smaller than a print width of the printing head, whereby it is possible to make inconspicuous stripes and density unevenness in an image caused by variation in the ejecting characteristics of each printing element and in a conveyance amount.
Incidentally, in the case of performing such multi-pass printing, it is conventionally common to determine print data for each printing scan by performing logical AND between binary data for each pixel in which printing (1) or non-printing (0) of a dot is specified and a plurality of mask patterns. More specifically, the mask patterns for printing scans in which allowance (1) or non-allowance (0) of dot printing is specified for each pixel are prepared such that the mask patterns have a complementary relationship. Dots to be actually printed by each printing scan are determined by performing logical AND between the plurality of mask patterns and binarized image data. In the multi-pass printing using the mask patterns, even if image data to be printed is divided into a plurality of printing scans, dots can be printed while an exclusive and complementary relationship among the printing scans is maintained. Accordingly, stripes and density unevenness can be inhibited without impairing density information which the image data has.
However, in the above multi-pass printing, groups of dots printed by the plurality of printing scans have a completely exclusive relationship among each other. Accordingly, in a case where a conveyance error occurs abruptly, displacement among groups of dots may change the coverage of dots on a print medium and cause density unevenness.
Japanese Patent Laid-Open Nos. 2000-103088 and 2011-000801 disclose a method for reducing the above adverse effects on an image, including: dividing multi-valued image data before binarization into a plurality of portions corresponding to individual printing scans; and binarizing each of the divided multi-valued image data by error diffusion methods independently without correlation.
FIGS. 28A and 28B show a dot arrangement in a case where multi-pass printing is performed by using masks having a complementary relationship (FIG. 28A) and a dot arrangement in a case where multi-pass printing is performed by the method disclosed in Japanese Patent Laid-Open Nos. 2000-103088 and 2011-000801 (FIG. 28B) for the sake of comparison. For the sake of simplicity, the figures show a case of 2-pass multi-pass printing that is performed by two printing scans to complete an image. In the figures, a black circle 41 represents a dot printed by a first printing scan, a white circle 42 represents a dot printed by a second printing scan, and a grid-pattern circle 43 represents a dot printed in an overlapping manner by the first and second printing scans.
In a case where multi-pass printing is performed by using mask patterns having a complementary relationship, dots printed by the first printing scan and dots printed by the second printing scan are in exclusive positions. Accordingly, as shown in FIG. 28A, there is no dot (grid-pattern circle) printed in an overlapping manner by the two printing scans. On the other hand, in a case where the multi-valued image data is divided into for the first printing scan and the second printing scan and thereafter the divided data are binarized respectively as disclosed in Japanese Patent Laid-Open Nos. 2000-103088 and 2011-000801, without correlation, and the dots printed by the first printing scan and the dots printed by the second printing scan do not have an exclusive and complementary relationship. In other words, as shown in FIG. 28B, there are dots (black circles and white circles) printed only by one of the two printing scans, but there are some overlapping dots (grid-pattern circles) printed in an overlapping manner.
A case where a conveyance error of one pixel occurs in a conveyance operation between the first printing scan and the second printing scan will be considered below. In the case of FIG. 28A, the group of the white circles is shifted one pixel in the same direction relative to the group of the black circles, and the black circles and the white circles completely overlap each other. As a result, the coverage of dots on the print medium decreases greatly, and the density of the image decreases as well. In a case where an area in which printing position displacement between the first printing scan and the second printing scan occurs, exists among areas in which printing position displacement does not occur, density unevenness is recognized. On the other hand, in the case of FIG. 28B, there are areas in which a black circle and a white circle overlap each other, but there are also areas in which a black circle or a white circle separated from an overlapping dot (grid-pattern circle). As a result, the coverage of dots on the print medium does not change greatly, and accordingly, the density of the image changes little.
As stated above, density unevenness caused by printing position displacement between printing scans can be inhibited by previously preparing dots to be printed in an overlapping manner by a plurality of printing scans based on Japanese Patent Laid-Open Nos. 2000-103088 and 2011-000801.
Incidentally, the printing method disclosed in Japanese Patent Laid-Open Nos. 2000-103088 and 2011-000801 can also be applied to a full-line type printing apparatus having a plurality of printing heads for ejecting inks of different colors. In a case where in the full-line type printing apparatus having the plurality of printing heads, a mounting error between printing heads, variation in the speed of scanning a print medium, or the like occurs, there occurs displacement of the position where each printing head prints the print medium, and the overlapping state of dots becomes unstable. As a result, there is a case where the coverage of dots on the print medium changes, and the desired density or hue of the image cannot be obtained.
Even in such a case, it becomes possible to establish a position relationship between dots printed by the printing heads as shown in FIG. 28B by quantizing print data for the plurality of printing heads by different error diffusion methods as disclosed in the above patent literatures. As a result, it becomes possible to inhibit density variation and hue variation caused by printing position displacement.
However, the configuration disclosed in Japanese Patent Laid-Open Nos. 2000-103088 and 2011-000801 cannot optimize the number of overlapping dots although it is possible to print some overlapping dots. Accordingly, there is a case where since the number of overlapping dots is too small, the effect of reducing density unevenness is not sufficiently obtained, and there is also a case where since the number of overlapping dots is much larger than necessary, the density of a real image is not sufficient. Further, there is almost no concern that in a low gradation region in which the number of printed dots is small, density variation and hue variation are caused by dot printing position displacement, but there is a case where overlapping dots are formed in the low gradation region and deteriorate the graininess of the image.
The above phenomena are caused by performing error diffusion for divided image data in parallel without correlation. According to the configuration disclosed in Japanese Patent Laid-Open Nos. 2000-103088 and 2011-000801, the arrangement of dots on a print medium depends on error diffusion methods adopted for divided image data. However, these error diffusion methods are not designed in consideration of superimposing images after binarization again and printing the superimposed images, and it is impossible to control the dispersion of dots and the number of overlapping dots after printing, and the like.